Building a DIY ALDL Cable for 94-95 LT1 Cars

Hey all,

There has been other threads here, on the Impala SS forum, and other corners of the web, but I wanted to pull some of that info together, and share my experience. Perhaps it will be helpful to others. I am not a electronics expert, and I assume I am not in the minority here.

My cable, which I had for many years from Red Devil River, started acting up a few months ago. I had to put pressure on the adapter to make it connect to the computer, and after bricking a PCM as a result, I decided to make my own.


First, this is the post I referenced. Good info to start.
https://www.firebirdnation.com/forums/topic/457761-make-an-obd1-cable-for-under-20/#entry888714

Note that 94/95 cars should be a 12 pin OBD connector. If you aren't sure, count the pins under the dash.

Essentially, what you have to do is convert the serial data coming out of your obd port to something your computer can read through it's usb port.
FTDI is one of the most popular companies making these converters. What we are looking for is an FT232RL chipset. I went with this unit. it is easy to wire, has a protective case, and is on Amazon prime, so it comes quickly, as opposed to many of the cheaper ones you will find coming from China and abroad.

https://www.amazon.com/dp/B07BBPX8B8/ref=dp_prsubs_1


If you followed the link, you may have noticed that the board has a black, rectangular chip on it. This is what we need. I purchased one with a FT232BL chip, which is square, and it did not play well with TunerCat. It did strangely work with Scan9495, never figured out why tunercat did not work with it. Anyway.

Also it is to the best of my knowledge, a genuine FTDI Chip. Some lower cost ones are knock offs, and people have experienced issues with them.





Connect the RXD and TXD wires together, and wire it to Pin M on the above diagram. Wire the ground to pin A.
*NOTE: Do not try to solder to the pins in the board. You can destroy it. Trust me.*

You can use a paper clip, hardwire it, or cut up a cheap 12 pin obdI harness. Something like this would do.


https://www.ebay.com/itm/12Pin-ALDL-OBD1-to-16-Pin-OBD2-Connector-Adapter-Cable-for-GM-Automobile-Vehicle/253605448882?epid=597808185&hash=item3b0c100cb2:g:LjgAAOSwoBlaYcc5:rk:11&frcectupt=true

Make sure you download the proper driver from FTDI. They can be found here. Note, some claim counterfeit chips will self destruct with the updated driver. Should you choose to go with a cheap one, you may want to find version 24 and turn off automatic updates on the driver.
https://www.ftdichip.com/Drivers/VCP.htm

At this point, you should see a COM port appear on the Device Manager tab when you plug your cable in. When plugged into the car, an led on the board will start flashing, indicating it is talking.

Ta-da! You can now get on with programing, datalogging, or whenever you need to do.

Hopefully this is helpful to some of you. You should be able to save some money, and more importantly, learn something new!

What will work with the 91-93 OBD1?

I have yet to own a 91-93, but I believe the main change is that serial data is on pin E (can anyone confirm?). In that case all you have to do tie RXD and TXD to both pin E and pin M, then you should be able to scan both styles.

That is, supposing you have the software to do so. What are people using on early cars?

I would also strongly suggest hardwiring it into the back of your ALDL port if you are flashing the PCM using TunerCat or eehack. The ALDL pins are quite loose. I had a PCM ruined just from somebody slamming a door and jiggling the car while I was uploading a tune. I'm just going to hardwire the USB cable to the car.